Intensity control compensation circuit for use in a cathode ray oscilloscope



United States Patent 3,168,679 INTENSITY CUNTRQL COMPENSATIUN CIRUIT FOR USE IN A CATHODE RAY ()SQILLQSCGKE John R. Kobbe, Beaverton, 0reg., assignor to Teittronix, Inc, Beaverton, Greg, a corporation of Oregon Filed May 8, 1961, Ser. No. 108,322

r 11 (Claims. (CL 315-30) The present invention relates generally to electron beam display devices, and in particular to an intensity control compensation circuit for use with such electron display devices;

In an electron beam display device, such as a cathoderay tube used in a cathode-ray oscilloscope, it is conventional to provide an intensity control for varying the brightness of the light spot produced by the electron beam impinging on the fluorescent screen in such display device. Previously, this intensity control has been effected by varying the voltage difference between the cathode and the control grid of the display device usually by changing the DC. bias voltage appliedto the control grid. .This is satisfactory if accurately regulated supply voltages are supplied to the other electrod es of the tube such as the cathode, anodes, andother control electrodes including the focusing electrode and the deflection blanking and unblanking plates employed in the tube chosen to illustrate the present invention. The DC. voltages applied between certain of the electrodes of such tubes are usually of the order of several thousand volts. Such high voltage sources ordinarily have internal impedances of several thousand ohms and when the current flowing through the external circuits connected to the various electrodes is varied, as a result of varying the grid bias relative to the cathode, the voltages applied to the various electrodes also vary in substantial amounts in absence of the voltage regulation discussed above. Such varying voltages vary the operating characteristics of the display device including the focusing eifects of the various electrodes in the tube as well as that of the focusing control and even more importantly varie the deflection sensitivity of the deflection system of the tube. Such deflection sensitivity varies inversely with the speed of the electrons in the electron beam and variations of the voltages referred to above varies the electron speed.

Accurate voltage regulation of high voltage sources require considerable additional circuitry and adds complication, weight and expense to electron beam display devices. In order to overcome the above discussed disadvantages without the necessity of such voltage regulations, the present invention provides an intensity control compensation circuit for varying the control grid bias voltage to change the electron beam spot intensity while leaving the voltages applied tothe other electrodes of the tube substantially unchanged. Such intensity control compensation circuit varies the voltage on the control grid with respect to that onthe cathode so as to change the current from the high voltage source and simultaneously changes the value of resistance in the supply circuit from the high voltage source to compensate for a change in voltage drop in such source and maintain the voltages on the other electrodes substantially constant.

Therefore one object of the present invention is to provide a means for varying the electron beam spot light intensity in an electron beam display device supplied from an unregulated source of high DC. voltage Without substantially affecting the other operating characteristics of such display device.

Another object of the present invention is to provide an intensity control compensation circuit for an electron beam display device including a control for varying the voltages applied between the control grid and the cath- Bdhhfilh Patented Feb. 2, 1965 ode in such display device while simultaneously varying the resistance in the supply circuit between such cathode and a high voltage unregulated D.C. source having internal resistance, in order to vary the intensity of the light spot formed by the electron beam on the fluorescent screen in such display device Without substantially affecting the voltages applied to the other electrodes of the tube.

Still another object of the invention is to provide an intensity control compensation circuit for the cathode-ray tube in a cathode-ray oscilloscope, including a variable resistor for varying the voltage between the cathode and the control grid of such cathode-ray tube and at the same time varying the resistance in series between such cathode and an unregulated DC. source supplying high voltage to said cathode in order to maintain'substantially constant the voltage applied to the cathode to thus change the intensity of the light spot formed-by the electron beam on the fluorescent screen of said such cathode-ray tube without substantially aifecting the operating characteristics of such tube including the focusing of such spot and the deflection sensitivity of the deflection system of such tube.

Additional objects and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment of the present invention shown in the accompanying drawing of which:

=FiG. 1 is a schematic diagram showing a cathode-ray tube and an intensity control compensation circuit in accordance with the present invention,

FIG. 2 is a graphical representation of the variation of the voltages on the cathode, control grid and focusing electrode of the cathode-ray tube when the intensity con trol is varied in the circuits of FIGS. 1 and 3; and

FIG. 3 is a fragmentary view similar to a portion of FIG. 1 showing a prior art intensity control circuit.

The intensity control compensation circuit of FIG. 1 is shown in connection with a cathode-ray tube 10 having a fluorescent screen 12, vertical deflection plates 14, horizontal deflection plates '16, an acceleration anode 1-8, an astigmatism correcting electrode 2%, focusing electrode 22, a deflection blanking structure 24, a control grid 26, a cathode 2'8, and a cathode heater 29. The cathode heater 29 may be connected across a source of AC. or DC. heater voltage. The cathode 28 may be connected to a source 31 of high negative DC. voltage through a variable intensity control resistor 30 and a grid bias voltage limiting resistor 332 with a bypass capacitor 34 connecting the cathode 28 to ground. The source 31 may be a conventional rectifier and filter circuit which has a substantial internal resistance and an unregulated DC. output voltage although it may be supplied with a regulated AC. input voltage. The control grid as is maintained negative with respect to cathode 28 by connecting it between voltage divider resistors 36 and Edwhich are connectedin parallel with intensity control resistor 30 and limiting resistor 32. The control grid 26 may also be connected to a Z axis or intensity modulation input 4d through an R.C. coupling impedance including a coupling capacitor '42 and coupling resistor 44.

A string of voltage dropping resistors 4s, 4d, 59 and 52 and a focusing potentiometer 54 are connected between the source 31 of high negative DC. voltage and ground in series with the previously described seriesparallel connection of the intensity control resistor 36, limiting resistor 32 and voltage divider resistors 36 and 38. The focusing electrode 22 is connected to the movable contact 56 of the focusing control potentiometer 54 so that its voltage may be adjusted by the movement of such contact. The astigmatism correcting electrode 20 is connected through the movable contact of an astigmatism control potentiometer 53 to a source of positive DC. bias voltage and accelerating anode 18 is connected between voltage divider resistors 69 and 62 also to a source of positive DC. bias voltage. The two pairs of vertical deflection plates 14 and horizontal deflection plates 16 may be connected to conventional sources of a signal voltage and a time base voltage respectively, not shown, for electrostatic deflection of the electron beam.

Blanking of the electron beam emitted from the cathode 28 during the time of its return trace on the fluorescent screen 12 may be accomplished by means of the deflection blanking structure 24 which may include two pairs 63 and 64- of cross-connected deflection plates with the plates of each pair positioned on opposite sides of the electron beam. One plate of each pair of deflection plates 63 and 64 is connected to a source of positive D.C. blanking voltage through a voltage divider including resistors 65 and 66 and to ground through a bypass capacitor 63. The other deflection plate of each pair of plates as and 64 is connected to a source of positive unblanking voltage ulses through input terminal to and are normally at a blanking potential. In the absence of an unblanking pulse, the pair of plates 64 deflects the beam to an extent that it strikes an element in the deflection structure 24 and does not reach the screen 12. A positive unblanking pulse of approximately the value of the voltage difference between the two plates 64 will however decrease the deflection produced by the pair of plates 64 so that the beam continues through the blanking structure 24 to stnike the screen 12 and is thereby unblanked. The voltage between the pair of deflection plates 63 reverses the deflection produced by the voltage between the deflection plates es so that undesired movement of the spot on the screen is prevented during either the blanking or the unblanking operation (i.e. during the transition period between the condition where the defiection voltage between plates 64 is sufficient to cause the beam to be blanked and the condition where there is substantially no deflection voltage between either pair of plates).

The operation of a specific example of an intensity control compensation circuit of the present invention may be more readily understood with reference to the graph shown in FIG. 2. In this graph the voltages on the cathode 28, the control grid 26 and the focusing electrode 22 as well as the output voltage of the source 31 are plotted against the grid bias voltage between the control grid 26 and the cathode 28, for both the circuit of FIG. 1 and a conventional intensity control circuit such as shown in FIG. 3 in which a potentiometer 73 is connected in series with the voltage source 31 and employed to vary the voltage on the control grid 26 with respect to the cathode 28. For purposes of explanation it will be assumed that the voltage source 31 has a no load output voltage of 3000 volts and an internal resistance of 161 kilohrns. It will also be assumed that the tube draws 485 microamperes of cathode current at a grid bias of 21 volts, is cut off at a grid bias of -60 volts, and that it has a typical cathode current-grid bias characteristic curve. It is also assumed that the intensity control resistor Etlis 200 kilohms, the limiting resistor 32 is 36 kilohrns, the voltage divider resistor 36 is 1.5 megohms, the voltage divider 38 is also 1.5 megohnis, the voltage dropping resistors 46, 48 and 5ft are each 820 lrilohms, the voltage dropping resistor 52 is 470 kilohrns, and the focusing resistance potentiometer 54 is 1 megohrn, and that the movable control 56 is set at the center position of the potentiometer. curves shown in solid lines in FIG. 2 result as the intensity control resistor 35) is varied from 0 to 200 kilohms. When the intensity control resistor 3th is set at 0 and the grid bias voltage is 21 volts with respect to the cathode, the cathode voltage V shown by curve 72, is 2768 volts while the control grid voltage V shown by curve 74, is -2789 volts due to the action of voltage divider The voltage resistors 36 and 38 so that the control grid 26 is 21 volts negative with respect to cathode 28 as mentioned above. As the resistance of variable resistor 39 is increased and the negative grid bias is increased, the cathode voltage shown by curve '72 becomes slightly more negative and then less negative until it returns to 2768 volts when the grid bias is at the cut off value of 60 volts which occurs when the resistance of the intensity control resistor 30 is approximately 145 kilohms. Also, as the resistance of the intensity control resistor 30 is increased the control grid voltage shown by curve '74 also becomes more negative and then less negative, but at a different rate than'that of the cathode voltage due to voltage divider resistors 36 and 38, so that it is approximately 2827 volts when variable resistor 30 is 145 kilohms thereby making the control grid 60 volts negative with respect to the cathode at cut off. As the intensity control resistor 30 is further increased beyond cut off, the grid bias increases to -76 volts when the resistance is 200 kilohms and both the cathode and grid voltages become less negative but the tube is not operating after 60 grid bias produces cut off so this is immaterial. Therefore, the effect of varying the resistance of the intensity control resistor 30 from 0 to 145 kilohms is to increase the voltage difference between the control grid 26 and the cathode 28 from 21 volts to 60 volts during operation of the tube which results in a corresponding variation in the intensity or brightness of the light spot produced on the fluorescent screen 12 of the cathode-ray tube by the electron beam emitted from cathode 28 and passing through control grid 26. As the resistance of the intensity control resistor 30 is varied from 0 to 145 kilohrns the cathode voltage 72 remains substantially constant so that in fact its variation is less than 0.9% of the average voltage thereon, and the focusing voltage V; on focusing electrode 22, shown by curve 76, also remains substantially constant.

Similar curves 78, 80 and 32 show in dotted lines the focusing electrode voltage V the cathode voltage V and control grid voltage V of the prior art circuit of FIG. 3 where the potentiometer '70 has a resistance of 106 kilohrns so as to give a similar grid bias variation from 21 volts to 76 volts, all other values remaining the same as in the circuit of FIG. 1. As shown in FIG. 2, the cathode voltage 80 of the uncompensated intensity control circuit of FIG. 3 varies from 2687 to 2810 volts during operation of the tube before cut off so that the cathode voltage variation is more than 4.4% of the average voltage on the cathode, and the focusing voltage 78 varies from 2024 to 2116 volts which is a voltage variation of more than 4.4% of the average voltage on the focusing electrode. The cause of this voltage variation is the fact that a change of the setting of potentiometer 70 results in a change in current flow through the internal resistance of the unregulated D.C. voltage source 31 so that there is variation in the internal voltage drop of such source. If this internal resistance is 161 kilohms, the source voltage V supplied to the external circuit varies from 2811 to -2886 volts during operation of the tube in either the circuit of FIG. 3 or the circuit of FIG. 1 as shown by the curve 84. One effect of the cathode voltage variation caused by the above is to change the voltage difference between the cathode 28 and acceleration anode 18 which changes the speed of the electrons accelerated by the anode 1% which in turn varies the deflection sensitivity of the horizontal deflection plates 16, the vertical deflection plates 14 and the blanking structure 24 causing distortion of the wave trace on the screen 12 of the cathode-ray tube. The effect of both the focus voltage variation and the cathode voltage variation isto change the voltage difference between the focus electrode 22 and the cathode 28 which causes defocusing of the electron beam spot on the screen 12 of the tube. Therefore, when the intensity of the electron beam spot is varied by potentiometer 70 in the uncompensated intensity control circuit of FIG. 3, other operating characteristics of the cathode-ray tube including the deflection sensitivity, deflection blanking and spot focusing are affected so that the performance of the tube is unsatisfactory when an unregulated source of D.C. voltage 31 is used. 7

As a result of the intensity control compensation circuit shown in FIG. 1, the intensity of the electron beam light spot may be varied without substantially affecting the other operating characteristics of the cathode-nay tube including the deflection sensitivity, the focusing of the spot and the blanking and unblanking of the electron beam. The use of the intensity control compensation circuit of the present invention in the cathode-ray tube of a cathode-ray oscilloscope having an unregulated source of high D.C. voltage therefore results in an oscilloscope whose operation is substantially unaifected by variations of the intensity control.

It is to be understood that the cathode-ray tube 10 of FIG. 1 is merely illustrative of cathode-ray tubes and that the intensity control compensation circuit of the present invention may be used with other electron beam display devices. Also it should be understood that details of the intensity control compensation circuit described above may be varied by those skilled in the art in an obvious manner Without departing from the spirit of the present invention. Therefor the preceeding detailed description of the preferred embodiment of the present invention is not intended to limit the scope of the invention which is to be determined only from the following claims.

I claim:

1. An electrical control circuit for an electron beam display device having a cathode and a control grid, said circuit comprising:

a source of unregulated voltage having internal resistance, means including a series resistance circuit having a plurality of resistors in series with each other and with said source for applying different voltages from said source to said cathode and said grid, and

means for varying the control grid voltage and for maintaining the cathode voltage substantially constant in spite of variations of the voltage of said voltage source due to variations of current flow through the internal resistance of said source to thus change the intensity of the light image formed by the electron beam emitted by said cathode without substantially affecting other operating characteristics of said display device,

the last named means including means for varying the value of resistance of one of said resistors of said series circuit, which is connected between said cathode and said voltage source.

2. An electrical control circuit for a cathode-ray tube having a cathode and a control grid, said circuit comprising:

a source of unregulated direct current voltage having internal resistance,

means including a series resistance circuit having a plurality of resistors in series with each other and with said source for applying different direct current voltages from said source tothe cathode andcontrol grid of said tube so that said grid is negative with respect to said cathode, and

means for varying the grid voltage and for maintaining the cathode voltage substantially constant in spite of variations of the voltage of said voltage source due to variations of current flow through the internal resistance of said source to thus change the intensity of the light spot formed on the fluorescent screen of said ,tube by the electron beam emitted by said cathode without substantially aifecting the other operating characteristics of said tube,

the last named means including means for varying the value of resistance of one of said resistors of said series circuit.

3. A control circuit for an electron beam display device having a cathode, a control grid and a focusing electrode comprising:

a source of unregulated voltage having internal resistance I 7 means including a series resistance circuit having a plurality of resistors in series with each other and with said source for applying different voltages from said source to said cathode, said control grid and said focusing electrode, and g means for varying the grid voltage and for maintaining the focusing electrode voltage and the cathode voltage substantially constant in spite of variations of the voltage of said voltage source due to variations of current flow through the internal resistance of said source to thus vary the intensity of the light image of the electron beam emitted by said cathode while maintaining said image in focus,

the last named means including means for varying the value of resistance of one of said resistors of said series circuit.

4. A control circuit for a cathode-ray tube having a cathode, a control grid and a focusing electrode comprising: 1

a source of unregulated direct current voltage having internal resistance,

means including a series resistance circuit having a plurality of passive i'esistors for applying dfiferent negative direct current voltages from said source to the cathode, control grid and focusing electrode of said tube so that said grid is negative with respect to said cathode and said focusing electrode is positive with respect to said cathode, and

means for varying the grid voltage and for maintaining the focusing electrode voltage and the cathode voltage substantially constant in spite of variations of the voltage of said voltage source due to variations of current flow through the internal resistance of said 'source tothereby vary the intensity of the light spot formed on the fluorescent screen of said tube by the electron beam emitted by said cathode without substantially affecting the other operating characteristics of said tube including the focusing of said spot and the deflection sensitivity of said tube,

the last named means including means for varying the value of resistance of one of said resistors of said series circuit.

5. A control circuit for a cathode-ray tube having an anode, a cathode, a control grid, a focusing electrode, a fluorescent screen and deflection plates, said circuit comprising:

means to apply an acceleration voltage to said anode to accelerate an electron beam in said tube,

means for applying deflection voltages to said deflection plates to deflect said electron beam before it reaches said fluorescent screen,

means including a series resistance circuit having a plurality of resistors for applying different voltages from a source of unregulated voltage to said cathode,

control grid and focusing electrode, and

means for varying the grid voltage independently of said acceleration voltage and said deflection voltages and for maintaining the cathode voltage and the focusing electrode voltage substantially constant in spite of any variations of the voltage of said unregulated voltage source due to variations in current flow through said' source to thus vary the intensity of the light spot formed by said electron beam on said fluorescent screen without defocusing said spot and without changing the deflection sensitivity of said tube,

the last named means including means for varying the value of resistance of one of said resistors of said series circuit. 6. A control circuit for a cathode ray tube having an anode, a cathode, a control grid, a focusing electrode, deflection plates, deflection blanking plates and a fluorescent screen, said circuit comprising:

means to apply a direct current acceleration voltage to said anode to accelerate the electron beam therein,

means to apply blanking and unblanking voltages to said electron beam blanking plates to control by deflection blanking when the electron beam reaches the fluorescent screen in said tube,

means for applying deflection voltages to said deflection plates of said tube in order to deflect the electron beam before it strikes said fluorescent screen,

means including a series resistance circuit having a plurality of resistors for applying different direct current voltages from an unregulated voltage source 'to said cathode, control grid and focusing electrode,

and

means for varying the control grid voltage independently of said acceleration voltage, said blanking and unblanking voltages and said deflection voltage and for maintaining the cathode voltage and focusing electrode voltage substantially constant in spite of any variations of said unregulated voltage source due to variations in current flow through said source to thus vary the intensity of the light spot formed on the fluorescent screen in said tube by the electron beam emitted by said cathode without affecting the other operatingcharacteristics of said tube including the focusing of said spot and the deflection sensitivity of said blanking plates and said deflection plates,

the last named means including means for varying the value of resistance of one of said resistors of said series circuit. 7

7. An electrical control circuit for a cathode-ray tube having a cathode, a control grid and a fluorescent screen, said circuit comprising:

a source of unregulated voltage,

and means for varying the intensity of a light spot formed on said screen including a variable impedance connected in series between said voltage source and said cathode and forming part of a voltage dropping resistance circuit,

said means also including a voltage divider impedance connected in parallel with said variable impedance, and having said control grid connected between portions of said voltage divider impedance for causing said variable impedance to vary the voltage applied to said control grid by said voltage source and for maintaining substantially constant the voltage applied to said cathode by said voltage source to thereby vary the intensity of the light spot without substantially affecting the other operating characteristics of said tube.

8. An electrical control circuit for a cathode-ray tube having a cathode, a control grid and a fluorescent screen, said circuit comprising:

a source of unregulated direct current voltage having internal resistance,

and means for varying the intensity of a light spot formed on said screen including a variable resistor connected in series between said voltage source and said cathode and forming part of a voltage dropping resistance circuit, and

said means also including a pair of series connected voltage divider resistors connected across said variable resistor and having said control grid connected between said voltage divider resistors for causing said variable resistor to vary the voltage applied to 8 said control grid from said voitage source and for maintaining substantially constant the voltage applied to said cathode by said voltage source in spite of the variation of said voltage source due to the change in current flowing through said internal resistance to thereby vary the intensity of said light spot without substantially affecting the other operating characteristics of said tube including electron beam deflection sensitivity.

9. A control circuit for a cathode-ray tube having a cathode, a control grid, a focusing electrode and a fluorescent screen, said circuit comprising:

a variable impedance connected in series between a source of unregulated voltage and said cathode and forming part of a voltage dropping resistance circuit,

a voltage divider impedance connected in parallel with said variable impedance and having said control grid connected between portions of said voltage divider impedance, and

said focusing impedance connected to said focusing electrode in series with said variable impedance and said voltage divider impedance so that variation of said variable impedance results in a change of the control grid voltage supplied by said voltage source while both cathode voltage and focusing electrode voltage are maintained substantially constant, thereby enabling variation of the intensity of the light spot formed on the fluorescent screen of said tube by the electron beam emitted by said cathode without affecting the other operating characteristics of said tube including the focusing of said spot and electron beam deflection sensitivity.

10. A control circuit for a cathode-ray tube having a cathode, a control grid, a focusing electrode and a fluorescent screen, said circuit comprising:

a source of unregulated direct current voltage having an internal resistance,

a variable resistor connected between said voltage source and said cathode and forming part of a voltage dropping resistance circuit,

a voltage divider resistor connected in parallel with said variable resistor and having said control grid connected between portions of said voltage divider resistor, and

a focusing resistor connected to a focusing electrode in said tube in series with said variableresistor and said voltage divider resistor so that variation of said variable resistor results in a change of the control grid voltage supplied by said voltage source while both the cathode voltage and focusing electrode voltage are automatically maintained substantially constant in spite of the variations of said voltage source due to the change in current flow through said internal resistance, thereby enabling variation of the intensity of the light spot formed on said fluorescent screen by the electron beam emitted by said cathode without affecting the other operating characteristics of said tube including the electron beam deflection ensitivity and the focusing of said spot.

11. A compensated intensity control circuit for a cathode-ray tube comprising:

a cathode-ray tube having electrodes including a cathode, a control grid and a focusing electrode,

a source of unregulated direct current voltage,

a variable rheostat connected between a source of unregulated direct current voitage and said cathode,

a limiting resistor connected in series with said rheostat and said voltage source,

a pair of series connected voltage divider resistors connected in parallel with said rheostat and said limiting resistor and having said control grid connected between said voltage divider resistors, and t a variable resistance potentiometer, having its movable contact attached to said focusing electrode, said potentiometer being connected in series with the ,-;va-riation of said rheostat changes the grid voltage 3,168,679 A a r P '9" 1Q series-parallel connection of said rheostat, said limitcharacteristics of said tube including the focusing of ing resistor and said yoltage divider resistors so that said spot and the electron beam deflection sensitivity.

while the cathode yoltage and the focusing electrode 4 References Cl'ted m the file of this patent voltage are automatically maintained substantially 5 UNITED STATES T I constant in spite of the resulting variations of said 2,371,897 Knick Mar. 20, 1945 voltage source, thereby enabling variation of the in- 2,939,042 Fathauer May 31, 1960 impinging upon the fluorescent screen in said/tube tenslty of the light spot formed by the electron beam 7 FOREIGN A S.

without substantially affecting the other operating 1 833,907 France 1938 

1. AN ELECTRICAL CONTROL VOLTAGE HAVING INTERNAL REDISPLAY DEVICE A CATHODE AND A CONTROL GRID, SAID CIRCUIT COMPRISING: A SOURCE OF UNREGULATED VOLTAGE HAVING INTERNAL RESISTANCE, MEANS INCLUDING A SERIES RESISTANCE CIRCUIT HAVING A PLURALITY OF RESISTORS IN SERIES WITH EACH OTHER AND WITH SAID SOURCE FOR APPLYING DIFFERENT VOLTAGES FROM SAID SOURCE TO SAID CATHODE AND SAID GRID, AND MEANS FOR VARYING TH CONTROL GRID VOLTAGE AND FOR MAINTAINING THE CATHODE VOLTAGE SUBSTANTIALLY CONSTANT IN SPITE OF VARIATIONS OF THE VOLTAGE OF SAID VOLTAGE SOURCE DUE TO VARIATIONS OF THE VOLTAGE OF SAID THROUGH THE INTERNAL RESISTANCE OF SAID SOURCE TO THUS CHANGE THE INTENSITY OF THE LIGHT IMAGE FORMED BY THE ELECTRON BEAM EMITTED BY SAID CATHODE WITHOUT SUBSTANTIALLY AFFECTING OTHER OPERATING CHARACTERISTICS OF SAID DISPLAY DEVICE, THE LAST NAMED MEANS INCLUDING MEANS FOR VARYING THE VALUE OF RESISTANCE OF ONE OF SAID RESISTORS OF SAID 